Coaxial electrical line attenuator



Jan. 24, 1967 H. E. STEVENS ETAL 3,300,737

COAXIAL ELECTRICAL LINE ATTENUATOR 2 Sheets-Sheet 1 Filed March 10, 1965 m mm INVENTORS Harold E.Sievens BY Leo Lesyk dawn 411 ,4 M4/ ATTORNEYS 2 SheetsShee i 2 H. E. STEVENS ETAL COAXIAL ELECTRICAL LINE ATTENUATOR Jan. 24, 1967 Filed March 10, 1965 INVENTORS Harold ESTevens BY Leo Lesyk ATTORNEYS United States Patent Orifice 3,3WJ37 i atented Jan. 24, 1&6?

3,309,737 COAXIAL ELECTRICAL LINE ATTENUATOR Harold Ellsworth Stevens, Lyndhurst, and Leo Lesyk,

Cleveland, Ohio, assignors to Bird Electronic Corporation, Solon, Ohio, a corporation of Ohio Fiied Mar. 10, 1965, Ser. No. 438,486 6 Claims. (Cl. 333--22) This invention relates to electrical devices for use in high frequency electrical apparatus and, more particular- 1y, to an improved high frequency reflectionless termination for a coaxial transmission line.

Frequently, in testing transmitter apparatus or in measuring radio frequency power, it is necessary to terminate a coaxial transmission line in a substantially reflectionless termination, or dummy load. This termination must be capable of absorbing and dissipating this power in the form of heat. The problem of providing a coaxial line reflectionless termination is very complex when the termination must dissipate power in the order of kilowatts. It is well known in the art that a coaxial line has predetermined physical dimensions which determine the electrical characteristics which must be matched by the termination in order to prevent the undesirable reflection of radio frequency waves from the termination. To achieve maximum reflection and maximum power transfer from a transmission line to a termination, the load should have a characteristic impedance which is equal to the characteristic impedance of the line. For coaxial lines, the characteristic impedance is defined by:

Zane- 1 r gle Where For maximum power transfer from a coaxial line, there should be a minimum of power reflection from the termination. Still further, any coolant material introduced into the termination must neither materially affect the voltage standing wave ratio, VSWR, nor physically affect the electrical components. These electrical requirements limit the diameter of the conductors of the terminations and therefore preclude the construction of large diameter terminations in an eflort to handle extremely large amounts of heat generated in the terminations. This physical requirement and the propensity of moisture to cause arcing between high voltage conductors preclude the use of water between the conductors.

Of the known forms of terminations employed for coaxial lines, some of the most satisfactory devices employ a tapered outer conductor, or horn chamber, which is connected to the outer conductor of the coaxial line and tapers logarithmically inwardly and contacts a resistor type cylindrical inner conductor which is connected to the inner conductor of the coaxial line. Thus, because of the logarithmic taper and because the horn must be connected to the resistor, this combination of horn and resistor type inner conductor restricts the length as well as the diameter of the resistor which may be employed. One example of line termination of the tapered horn-resistor type is found in Waterfield et al., Patent No. 2,946,005. Other examples of this general type of line termination are found in Bird et al., Patent No. 2,752,572 and Bird, Patent No. 2,556,642.

Terminations suitable for use with pulse type generators must be capable of withstanding repeated thermal shock due to the sudden application and removal of power from the transmission line.

Further, for maximum power handling capacity, as well as thermal shock capacity, the termination should be provided with a coolant system including means for maintaining the resistive surface at a uniform temperature or within very small differentials throughout the surface area. Water, which is economical and has a high heat of vaporization, is one of the best coolants by which maximum power handling capacity and thermal shock capacity might be achieved, but even a small amount of moisture between conductors which are subject to high voltage may cause arcing and the resulting destruction of the termination. Accordingly, an important consideration of this invention is the prevention of condensation of the opposed surfaces of the coaxial termination conductors.

Another important consideration in the utility of a line termination is its frequency range. The broader the range over which the termination will operate, the more versatile will be its application.

Accordingly, an object of this invention is to provide an improved coaxial line reflectionless termination having a relatively broad frequency range.

Another object of this invention is to provide an improved termination for a coaxial line which is constructed to utilize the heat of vaporization of water while avoiding the disadvantages previously encountered when employing water.

Yet another object of this invention is to provide a relatively small coaxial line termination capable of handling power of the order of kilowatts.

Still another object of this invention is to provide a coaxial line termination capable of withstanding repeated thermal shock.

It is a still further object of this invention to employ in a coaxial line termination, a hollow cylindrical member having resistive material coated on its outer surface and provision for fluid flow on the interior of the member, which member is formed of dielectric material having a high thermal conductivity. Thus, the member acts as an insulator to electrical currents while acting as a metal to thermal currents.

It is also an object of this invention to provide for gas under pressure on the exterior surface of the resistor to thus prevent condensation of moisture on the surface of the resistor when the resistor is not in use.

It is a still further object of this invention to provide a coaxial line termination capable of dissipating relatively high power by employing water as a coolant in which the presence of the water in the termination produces substantially no change in the voltage standing wave ratio as compared to the same termination without water to thus assure that the termination is capable of operating over, a wide frequency range, and in which good heat transfer between the resistor substrate and the water is obtained by producing turbulence in the water contacting the substrate.

It is yet another object of this invention to provide a coaxial line termination capable of dissipating power in the order of kilowatts over a wide frequency range.

Briefly, in accordance with aspects of this invention, we employ in a reflectionless coaxial transmission line termination a horn chamber serving as an outer conductor termination and as a terminating resistor housing and a resistor to be connected to the inner conductor of the transmission line, which resistor includes a resistive coating on a ceramic cylinder. Preferably, the resistor has an outer diameter substantially equal to the outer diameter of the inner conductor of the coaxial line. Advantageously, this ceramic cylinder is formed principally of a dielectric material such as beryllia, or BeO, which has a very high thermal conductivity and is a good electrical insulator. Also advantageously, the resistor is formed of resistive material coated on the outer surface of the ceramic cylinder, or substrate, which arrangement facilitates heat transfer from the resistor. By circulating water against the interior of the ceramic cylinder by producing turbulence in the water contacting the substrate and increasing the flow rate as the temperature of the water increases, heat is removed at a very rapid rate from the resistor. Thus, it is not necessary for the water to contact the resistive coating of the resistor where it might otherwise cause arcing to other portions of the termination and where it might react with the resistive coating. Advantageously, a perforated horn is inclosed within a fluid tight housing and a dry gas is introduced to the interior of the housing to purge any moisture entrapped during assembly and is utilized to prevent condensation on the resistor when it cools between periods of operation. The dry gas prevents condensation of moisture on the film resistor which might take place if the housing were filled with air containing water vapor.

To increase the power handling capabilities of the termination, a coolant system is provided which includes a fluid coolant conduit on the interior of, and preferably coaxial with, the cylindrical resistor. This conduit has apertures in the end there of adjacent the end of the coaxial line through which apertures a coolant, such as water, is forced. This coolant flows across the interior surface of the resistor and out through the end wall of the housing without either touching the resistive surface of the resistor or entering the space between the resistor and horn-shaped outer conductor. In accordance with other aspects of this invention, the fluid coolant conduit is stepped, or provided with sections of increasing diameter, in a direction from the inner conductor of the coaxial line outwardly toward the exterior of the housing to produce maximum heat transfer. The steps are weighted toward a smaller area in the chamber adjacent the discharge end to produce a greater velocity at this end, where the water is hottest, than the mean velocity, thus gaining a reduction of boundary temperature change.

In accordance with still other aspects of this invention, we employ, as a support for a resistive film, a ceramic material exhibits a relative high thermal conductivity, i.e., one which has a thermal conductivity, k, greater than .1 and preferably of the order of .49 calories per second per square centimeter per centimeter (traveled) per degree Centigrade (A+). This latter figure corresponds to the thermal conductivity of aluminum, which is approached by good beryllia. The ceramic material, however, acts as a dielectric to electrical currents. By employing this material as a supportfor a resistive film, it is possible to circulatewater on one surface of the material while the opposite surface of the material supports the resistive film and-thus the water rapidly removes the heat generated in the film by the high frequency electrical energy.

In accordance with more specific aspects of this invention, we employ in a line termination a cylinder consisting essentially of beryllia, or beryllium oxide as it is sometimes called, which has a thermal conductivity greater than .1 and preferably in the range of .2 to .492. Beryllia having this thermal conductivity is available commercially from either National Beryllia, Haskell, New Jersey or Coors, Golden, Colorado. With this novel arrangement, it is possible to utilize water and thus economically dissipate the heat generated by the high frequency power by obviating the use of other and more expensive coolants, such as oil.

In accordance with still other aspects of this invention, we employ, in a line terminating device inclosed .in a housing, a resistor formed by applying a resistive coating to one surface of a hollow ceramic member having a high thermal conductivity, means for supplying cooling liquid solely to the interior of the resistor, and means for supplying gas under pressure between the conductors of the line terminating device. This combination prevents condensation of moisture on the resistive material when no power is applied to the terminating device.

In accordance with still other aspects of this invention, we employ, in an inclosed line terminating device, a resistor formed of a cylindrical ceramic member and a resistive coating on the outer surface of the ceramic member, which ceramic member is formed of material having a high thermal coefficient. Advantageously, we provide this termination with a cooling system including means for circulating a liquid coolant such as water on the interior surface of the ceramic member which means includes turbulence producing means for producing turbulence in the water contacting the ceramic member. This termination includes assembly housing means for maintaining dry gas under pressure around the exterior surface of the resistor. The presence of the dry gas on the exterior of the resistor prevents condensation of moisture on the resistive coating. The liquid coolant circulating system advantageously includes an annular fluid conduit member which is axially aligned with the resistor and mounted on the inside .thereof and having a stepped outer surface which is stepped in a plurality of cylindrical sections of progressively increasing diameter, increasing toward the outlet of the annular passage between the fluid conduit and the resistor.

The embodiment of the invention illustrated in the accompanying drawings, forming a part of this specification, represents the best known mode. of practicing the invention for elfectuating the above objectives and certain other objectives having to do with details of construction and arrangements of parts which will become apparent as the description proceeds, this description being made by the use of reference numerals which indicate the parts throughout the several views.

In the drawings: v

FIG. 1 is a view, in elevation, partly broken away and in section, of a line terminating device according to one illustrative embodiment of this invention, schematically illustrating the manner of connection to a radio frequency generator and to a heat exchange system;

FIG. 2 is an end view, partly in section, taken from the left hand end of FIG. 1;

FIG. 3 is an end view, taken from the right hand end of FIG. 1;

I FIG. 4 is a view, in section, taken along the line 4-4 of FIG. 1;

FIG. 5 is a fragmentary view, to a larger scale, of a portion of the embodiment of FIG. 1; and

FIG. 6 is a broken view showing a portion of the defvice this view being similarand enlarged relative to FIG. 1V to reveal internal construction.

FIG. 1 depicts a system for terminating a coaxial line from a signal generator, which system includes a reflectionless type of line termination device 12 and means for dissipating heat generated at the line termination. This view of the load is in elevation, partly in section, with other portions of the system shown in block and schematic form. A signal generator 10 is connected by coaxial line 11 to the load or termination device 12. The system also includes a heat exchanger 13 connected by means of pipes 14, 15 to the load 12 and to a pump 16 which circulates coolant liquid between the load and the heat exchanger by means of a pipe -17. The signal generator 10 provides signals at any frequency from DC. up into the microwave region and this generator may be a portion of a transmitter. It is understood that the signal generator may be of any well known type and it may be of the pulse type, such as employed in radar, to generate pulses at a relatively low repetition rate. The line termination, according to this invention, is adapted to withstand thermal shock and is, therefore, suited-to opera ate as a terminating device to receive the power transmitted from a radar type transmitter.

The input ends of the inner and outer conductors (to be later described in detail) which comprise the energy dissipating section of the termination or load device are connected to the coaxial line 11 through a suitable connector section which may include an inner connector 18 and an outer connector 21. The coaxial line 11 (FIG. 1) includes an outer conductor 19 and an inner conductor 20 which are connected to such connector section of the load by conventional means. The outer conductor 19 is provided with a suitable fitting (not shown) received in an annular recess 23 in swivel follower 22, which follower is telescoped over reduced outer end of the tubular cylindrical outer connector 21. A swivel flange 26 is disposed about the ring 22 and includes a circular inwardly projecting radial lip 27 engaging behind annular shoulder projection 25 of the ring 22. The swivel flange 26 has a plurality of circumferentially spaced apertures 28 through which bolts may be inserted to engage a corresponding plate, not shown, on the end of the fitting on the outer conductor 19 of the coaxial line 11 to hold the plates together. The outer connector 21 abuts a circular shoulder provided by an internal rib 29 of the swivel follower or ring 22 and is secured thereto by brazing, indicated at 33. The swivel flange 26 includes an alignment pin 31 for engaging a suitable recess, not shown, in the fitting, also not shown, on the end of outer conductor 19. The end of outer conductor 21 remote from the ring 22 is connected to an apertured flange ring 35, the latter being formed with an internal rabbet to receive the connector and provide a radial locating shoulder 36 against which the connector 21 is abutted. This connection is secured by silver brazing around the periphery, as indicated at 37. The flange ring 35 abuts a companion flange ring 40 telescoped over one end of a cylindrical housing member 44. Silver brazing, indicated at 45, secures the flange ring 40 to the housing member 44. The flange rings 35, 40 are held together by a number of bolts 42 received through aligned openings. The housing member 44 is part of a gas tight enclosure which includes an end wall 47 in the form of an insulator of dielectric material such as polytetrafluoroethylene. This insulating end wall 47 is a circular ring having oppositely directed flat parallel surfaces. Its outer periphery is held between a shallow internal radial shoulder provided by an internal rabbet in the end of the cylindrical housing member 44 and the inner peripheral portion of the flat end face of the ring flange 35. A gas tight seal is defined by means of a rubber O-ring 48 confined in an annular recess 50 formed in the outer periphery of the plastic insulating member 47. A ring 51 of polytetrafluorine is held between connector 18 and end wall 47.

This termination is provided with means for maintaining a pressurized dry gas on the interior of the cylindrical housing to prevent the condensation of moisture on the surface of the resistor. This means is provided in the cylindrical housing 44 by a pair of apertures 53, 55. A gas conduit fitting 156 is threaded into aperture 53 and a gas pressure gauge 57 is coupled to member 44 by a pipe 58 which threadably engages aperture 55. A dry gas, preferably nitrogen, is introduced into conduit 56 from a suitable reservoir, not shown, to maintain the interior of the housing assembly under a suitable superatmospheric gas pressure. The gauge 57 is protected by means of a gauge cover 59 which is silver brazed at 60 to the outer surface of housing cylinder 44. Housing cylinder 44 is connected to a housing end plate 61 by silver brazing 62 to define a gas tight seal. A water chamber flange 63 abuts the housing end plate 61 and is secured thereto by means of bolts, such as bolt 64. An O-ring seal 65 is compressed in an annular recess 66 in the abutting face of the housing end plate 61 by water chamber flange 63 to define a gas tight seal.

The coolant system includes water chamber flange 63 which is connected to a water chamber tube 68 by means of silver brazing at 69. The end of water chamber tube 68 opposite flange 63 is defined by a water chamber plate 70 which is also brazed to water chamber tube 68 at '71. The chamber 72, defined by tube 68 and plates 63, 70, is the outer manifold chamber through which cooling liquid is exhausted from the coaxial line termination 12. The water chamber plate 70 has an aperture which receives a water inlet fitting 73. Water chamber tube 68 has another aperture 74 which is aligned with a water outlet fitting 75, which fittings are secured thereto by means of brazings 76, 77, respectively. Fittings 73, 75 are provided with suitable hose nipples 78, 79, respectively (shown in FIG. 1) which thrcadably engage the respective fittings and connect the respective coolant pipes 14, 17.

The electrical portion of the coaxial line termination includes a horn-shaped conductor which acts as a termination of the outer conductor 19 of the coaxial line and has an integral cylindrical extension 82 which snugly engages the inner surface of cylindrical housing member 44 and is welded, soft-soldered, or silver brazed thereto. Solder or braze is fed at spaced points 83 around the periphery of a full ring fillet 84, which solder feed holes are gas tight. The horn-shaped outer conductor has a plurality of apertures 85 therein for the purpose of permitting gas flow from the interior of the cylindrical hous ing member 44 to the interior of the horn member 80 for the purposes which will be subsequently described. The electrical portion of the coaxial line termination also includes a resistor assembly 88 which advantageously includes a ceramic dielectric cylinder 89 having a thermal conductivity, k, at least equal to .1 with a resistive coating 90 deposited, painted, or otherwise formed, on the outer surface of cylinder 89. A metallized terminal 91 is painted or otherwise formed on each end of cylinder 89 and overlaps each end of coating 90. Resistive coating 90 is electrically connected to the coaxial line connector section inner conductor 18 through terminal 91 and a contact member 92, having a sleeve-like portion which encircles the end of cylinder 89 and is soldered at 95, or otherwise joined, to resistor terminal 91 in liquid sealing relationship. The resistor contact 92 has a reduced cylindrical extension 94 which projects through the insulator 47 and compresses an O-ring 96 against an annular recess 97 in the insulator 47 to define a gas tight seal between the cylindrical extension 94 and the insulator 47. Resistor contact 92 includes a threaded extension 98 which passes through an end plate 99 of the connector section inner conductor 18 of the coaxial line 11 and is secured thereto by means of a split washer 100 and a nut 102.

In one embodiment, the dielectric cylinder is formed of beryllia with an outside diameter of 1%", and inside diameter of 1%", and is 12" long. The resistive coating 90 covers an area 10%" long. The resistance of this coating is of the order of 48.5 ohms to 51.5 ohms. Tests have shown that the line termination including this resistor assembly 88 are capable of successfully steadily dissipating power of the order of 50 kilowatts for several hundred hours.

As best seen in FIG. 5, the opposite end of the resistor assembly 88 from the resistor contact 92 has a contact 104 which has a sleeve portion 105 extending over the end of the ceramic cylinder 39 and soldered to the resistive terminal 91 by means of the solder indicated at 106. The end of horn 80 adjacent contact 104 is split, such as by sawing, and pressed into engagement with contact 104 by a plurality of split compression rings 107 to provide suitable contact pressure between the end of the horn 80 and resistor contact sleeve 105 in accordance with well known practice.

The resistor contact 104 passes through a relatively large aperture in the housing end plate 61 and a relatively small aperture in the water chamber flange 63 and compresses an O-ring 109 in an annular recess 110 of the water chamber flange to define a fluid tight seal between water chamber flange 63 and contact 104. The gas pressure on O-ring 109 reduces the pressure differential on the O-ring and is preferably greater than the liquid pressure in chamber 72.

The coolant system includes a coolant water tube 112 mounted in inlet water fitting 73 to compress an O-riug 114 in an annular recess 115 in inlet fitting 73 to define a fluid tight seal. The opposite end of water tube 112 is captivated by a recess 116 in the resistor contact 92. Advantageously, tube 112 is coaxial with the ceramic tube 89, as best seen in FIG. 4, such that the area between the two tubes is uniform in cross-section to provide a uniform path for the flow of coolant as will be subsequently described. A plurality of apertures 117 in tube 112 provides outlet passages for the coolant to permit the coolant to escape from the tube 112 into contact with the inner surface of the ceramic substrate resistor 88. Advantageously, the outer surface of tube 112 is formed with a plurality of steps 119, 120, 121, 122 and 123 by whichthe outer diameter of the tube 112 increases progressively in the direction of outlet manifold chamber 72 so that the flow rate of coolant increases as the coolant approaches chamber 72 where it is exhausted through the water outlet fitting 75.

Water tube 112 is preferably made of phenolic tubing or other dielectric material capable of withstanding hot water. The five steps: 119, 120, 121, 122 and 123 on the outer surface of tube 112 actually employed in the previously described operating embodiment are such that the outer diameter of the tube increases progressively from .76 inch to .92 inch in substantially equal increments and at evenly spaced intervals of 1% inches apart. The first step is spaced approximately 3.125 inches from the left hand end of tube 112. The interior diameter of the tube is uniform throughout the tube length and is of the order of .68 inch and the apertures 117 are formed in two rows of six apertures each, with one row rotated circumferentially 30 from the other row to provide ample outlet area for the water. In this illustrative embodiment, the apertures are oval shaped and have a major diameter of approximately inch. With this arrangement of elements and by maintaining water flow at a suitable rate within resistor assembly 88, the temperature of the resistive material may be held within a surface variation of :1" 0, thus permitting maximum power dissipation for the materials employed.

The gas, which is preferably dry nitrogen or other inert gas, is introduced through the conduit 56 and circulates around through the apertures 85 of the horn 80 and contacts the resistive coating 90, excluding air, thus to prevent the condensation of moisture on the resistive surface when no power is applied from generator 10. Advantageously, because the gas pressure is much greater than the pressure of the Water, any minute leakage which occurs in air seals will be outward. Inward leakage of air, and associated condensation at points where it is chilled below its dew point, is avoided. This leakage, if occurrent, will be indicated by gauge 57 and signal generator 10 can be turned off before damage occurs to the termination 12.

The ceramic material formed in the tube 88 has a very high thermal conductivity, k, defined in terms of calories per second per square centimeter per centimeter per degree Centigrade, preferably a value in the range corresponding to that of metals, such as aluminum and brass. The coeflicients of thermal conductivities for aluminum and brass are .49 and .26, respectively. For example, tube 88 may be formed principally of beryllia which is beryllium oxide and preferably has a thermal conductivity in the range of .20 to .49. Because of this high thermal conductivity, the heat generated in the resistor film 90 is rapidly conducted to the inner surface of tube 89 and is transferred to the water on the interior of tube 89. a

In one illustrative embodiment of this invention, ceramic substrate resistor 88 dissipates 30 kilowatts of radio frequency power and this resistor is approximately one foot long and has an outside diameter of 1.25 inches and the resistive film is a metal resistive film which is coated on the exterior surface of the ceramic cylinder for a distance of 10% inches and the ends of the resistive material are connected to the contacts 92, 104 by metallized terminals 91 suitably coated on both ends of the ceramic cylinder.

This embodiment was subjected to an accelerated life test in which it was shocked by the application and removal of power at 20% over the proposed rating of 25 kilowatts for 3,000 shock applications over a period of 50 hours. After this life test, the termination was dissembled and a close examination did not reveal any structural failures.

In accordance with the patent statutes, the principles of the present invention may be utilized in various ways, numerous modifications and alterations being contemplated, substitution of parts and changes in construction being resorted to as desired, it being understood that the embodiments shown in the drawings are given merely for purposes of explanation and illustration without intending to limit the scope of the claims to the specific details disclosed.

What we claim and desire to secure by of the United States is:

1. In a termination for a coaxial transmission line, the combination comprising:

a tubular ceramic dielectric member having a thermal coefficient at least equal to .2 calories per second per square centimeter per centimeter per degrees centigrade;

a film type resistor affixed on the exterior of said member;

means for connecting said resistor to the inner conductor of said line;

housing means enclosing said member in fluid sealing relationship; and

tubular conduit means for conducting a fiui-d to the interior of said member, said conduit means being within, substantially coextensive with, axially aligned with and spaced from said member and having a 1 plurality of steps of progressively larger diameter in the direction of fluid flow to produce increased turbulence and velocity in the exhaust flow of fluid therebetween.

2. An attenuating device for use in a high frequency coaxial electrical system, said device com-prising:

an elongated tubular outer conductor;

an elongated inner conductor within, substantially coextensive with and coaxial with the outer conductor, said conductors being separated by an annular dielectric filled space and each having a surface confronting the other across said filled space, one of the conductors comprising an elongated ceramic tube having an electrically resistive member on its surface and being formed of material having a coefiicien-t of thermal conductivity of the order of at least about .2 calories per second per square centimeter per centimeter per degrees centigrade;

fluid flow means disposed in fixed relation to said ceramic tube; and

conduit means connected to said one conductor and adapted to be connected also to a source of fluid coolant under pressure for supplying such fluid to the one conductor, said fluid flow means being adapted to constrain the flow of fluid so supplied to flow in one direction through said tube in a receiving run and in a reverse direction in a returning run, said receiving and returning runs both being separated from said dielectric space by the ceramic tube, said fluid flow means comprising a tubular member positioned coaxially within said Letters Patent ceramic tube, said tubular member having a stepped outer surface of increasing diameter which increases in the direction of said returning run.

3. An attenuating device for use in a high frequency coaxial electrical system, said device comprising:

an elongated, horn-shaped outer conductor;

an elongated inner conductor within and coaxial to said outer conductor, one of said conductors comprising an elongated ceramic tube having an electrically resistive means on one surface thereof and being formed of a material having a thermal conductivity of the order of at least .2 calories per second per square centimeter per centimeter per degrees centigrade;

fluid flow means disposed in fixed relation to said ceramic tube; and

conduit means connected to said fluid flow means and adapted to be connected also to a source of fluid coolant under pressure for supplying fluid to said one conductor, said fluid flow means being adapted to direct the flow of fluid supplied to said one conductor in one direction throughout said tube in a receiving run and in a reverse direction in a returning run said runs being concentric and within said ceramic tube, said conduit means including means spaced along said conduit means for producing turbulence in the fluid flowing in said returning run.

4. The combination according to claim 3, wherein said conduit means includes tubular member coaxially positioned within said ceramic tube and spaced therefrom and wherein said means for producing turbulence includes a stepped outer surface of progressively increasing diameter on said tubular member.

5. The combination according to claim 4 wherein the progressively increasing diameter of said tubular member increases in the direction of the returning run.

6. The combination according to claim 5 wherein said ceramic tube consists essentially of beryllium oxide.

References Cited by the Examiner UNITED STATES PATENTS 2,262,134 11/1941 Brown 333-22 2,421,758 6/1947 Ovrebo 33322 2,848,683 8/1958 Jones 33322 2,894,219 7/1959 Frederico 33322 3,044,027 7/1962 Chin et al. 333-22 3,213,392 10/1965 Hedberg 33322 3,241,089 3/1966 Treen 33322 FOREIGN PATENTS 1,181,935 1/1959 France,

HERMAN KARL SAALBACH, Primary Examiner.

R. F. HUNT, Assistant Examiner. 

3. AN ATTENUATING DEVICE FOR USE IN A HIGH FREQUENCY COAXIAL ELECTRICAL SYSTEM, SAID DEVICE COMPRISING: AN ELONGATED, HORN-SHAPED OUTER CONDUCTOR; AN ELONGATED INNER CONDUCTOR WITHIN AND COAXIAL TO SAID OUTER CONDUCTOR, ONE OF SAID CONDUCTORS COMPRISING AN ELONGATED CERAMIC TUBE HAVING AN ELECTRICALLY RESISTIVE MEANS ON ONE SURFACE THEREOF AND BEING FORMED OF A MATERIAL HAVING A THERMAL CONDUCTIVITY OF THE ORDER OF AT LEAST .2 CALORIES PER SECOND PER SQUARE CENTIMETER PER CENTIMETER PER DEGREES CENTIGRADE; FLUID FLOW MEANS DISPOSED IN FIXED RELATION TO SAID CERAMIC TUBE; AND CONDUIT MEANS CONNECTED TO SAID FLUID FLOW MEANS AND ADAPTED TO BE CONNECTED ALSO TO A SOURCE OF FLUID COOLANT UNDER PRESSURE FOR SUPPLYING FLUID TO SAID ONE CONDUCTOR, SAID FLUID FLOW MEANS BEING ADAPTED TO DIRECT THE FLOW OF FLUID SUPPLIED TO SAID ONE CONDUCTOR IN ONE DIRECTION THROUGHOUT SAID TUBE IN A RECEIVING RUN AND IN A REVERSE DIRECTION IN A RETURNING RUN SAID RUNS BEING CONCENTRIC AND WITHIN SAID CERAMIC TUBE, SAID CONDUIT MEANS INCLUDING MEANS SPACED ALONG SAID CONDUIT MEANS FOR PRODUCING TURBULENCE IN THE FLUID FLOWING IN SAID RETURNING RUN. 